The oxidation of formaldehyde in a flow reactor has been studied experimentally under diluted conditions at 800 to 1400 K and with the stoichiometry between fuel-rich and very lean. Of particular interest was the interaction of formaldehyde with nitrogen oxides. The results show that the oxidation of CH2O is initiated above 900 K, with the oxidation rate decreasing slightly with increasing [O-2]. Addition of NO under fuel-rich conditions inhibits oxidation, while under lean conditions a mutually promoted oxidation of CH2O and NO is observed at 750-1100 K. Based partly on recent accurate measurements of key reactions of CH2O and HCO, a detailed reaction mechanism for the oxidation of formaldehyde under post-flame conditions is proposed. It provides a good description of the experimental measurements as well as data from the literature. (C) 2003 The Combustion Institute. All rights reserved.